The policy, which takes effect immediately, comes three years after the organization removed its prohibition on gay youth, but local Boy Scout units chartered by religious organizations will still be permitted to exclude gay adults.

Students walk through metal detectors, the Metropolitan Opera searches the purses of patrons, and air travel involves long lines and security checks. But movie theaters are unlikely to introduce such stringent measures.

After notching a few legislative accomplishments this year, Congress is fast approaching a test, with the convergence of a series of fiscal challenges that if botched could lead to a government shutdown and even default.